updated module style of core.lua
updated module style for rima.lua, though there's still some issues...
moved sum.build into a new module 'rima.interface'
moved product.build into 'rima.interface'
It helps to save everything before a commit.  Addded changes to product.lua from last commit
moved case.build into 'rima.interface'
moved minmax.build_{min,max} into 'rima.interface'
moved func.build into 'rima.interface'
moved math ops into 'rima.interface'
moved index.define into 'rima.interface'
moved index.R into 'rima.interface'
reduced use of index:__index a little
reduced use of index:__newindex
moved __index and __newindex out of index and into expression
moved sets.ord and sets.range to rima.interface
moved sets.ref.pairs and ipairs to rima.interface